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<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by HooKooDooKu</i>
<br />The larger the pipe, the less pressure you lose, but the more it costs.
The only reason to ever telescope is to save money.
But pressure loss is a function of both pipe size AND flow rate (gpm).

Without a complex computer program, it's something of an art form to determine what size pipe to use where. But for starters, you need to determine what is your designed flow rate through each section of pipe. As a simple example, let's assume you have a zone with three heads in a line from the valve. The first and last heads are being designed for a flow of 2gpm and the one in the center, 4 gpm. So that means (regardless of the distances) we want 8gpm from valve to 1st head, 6gpm from 1st head to 2nd head, and 2gpm from 2nd to 3rd head. If the pipe from valve to 1st head is very long, perhaps a 1.5" pipe is called for to make sure there isn't too much pressure loss with an 8gmp flow rate. But perhaps the distance between the 1st and 2nd head is close enough that we can tollerate the pressure loss of using only a 1" pipe in this section were the flow is not estimated at 6gpm. But then the final leg between 2nd and 3rd head has such a low flow (dropping to only 2gpm) that we can tolerate the pressure loss of only a 1/2" pipe.

But how do you determine how much pressure loss you can tolerate? The number that I've heard (www.irrigationtutorials.com I believe) is that the pressure should be within 20% for all the heads on a circuit. In this example, if we are designing for 30psi at head #1, then the pressure at head #3 must be at least 24psi. So the size of the pipe from the valve to the 1st head must be sized so that the pressure is still about 30psi (after taking all the other pressure losses into account, such as water meter, backflow preventer, mainline, and valve). Then the combined pressure loss from head 1 to head 2 plus head 2 to head 3 must be less than 6psi.
an example, lest assume you have 3 heads on a strip. T
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From your example, I understand bigger pipe is usually the way to go for less pressure loss especially at long runs and friction loss from squarer bends e.t.c. I've designed the front two zones at 1 1/4" sched.40 piping that works wonderfully. Should I continue with 1 1/4" runs for the remainder of the yard then? I would really like to use poly pipe this time for the various bends e.t.c. as well.
What I was thinking i could do is run another main line to a new manifold box in the back yard and then split off some other zones once again. Also, the manifold in the front has room to expand for at least two more zones as well. I would like to run 1 1/4" laterals to the heads in the other zones for ease of install as well. Let me know if this sounds like a feasible solution. By the way, i have only about a 5-10 psi loss to the heads in my current zones as calculated with the 'dry' method in designing with my 40 psi from the well pump.

The larger the pipe, the less pressure you lose, but the more it costs.
The only reason to ever telescope is to save money.
But pressure loss is a function of both pipe size AND flow rate (gpm).

Without a complex computer program, it's something of an art form to determine what size pipe to use where. But for starters, you need to determine what is your designed flow rate through each section of pipe. As a simple example, let's assume you have a zone with three heads in a line from the valve. The first and last heads are being designed for a flow of 2gpm and the one in the center, 4 gpm. So that means (regardless of the distances) we want 8gpm from valve to 1st head, 6gpm from 1st head to 2nd head, and 2gpm from 2nd to 3rd head. If the pipe from valve to 1st head is very long, perhaps a 1.5" pipe is called for to make sure there isn't too much pressure loss with an 8gmp flow rate. But perhaps the distance between the 1st and 2nd head is close enough that we can tollerate the pressure loss of using only a 1" pipe in this section were the flow is not estimated at 6gpm. But then the final leg between 2nd and 3rd head has such a low flow (dropping to only 2gpm) that we can tolerate the pressure loss of only a 1/2" pipe.

But how do you determine how much pressure loss you can tolerate? The number that I've heard (www.irrigationtutorials.com I believe) is that the pressure should be within 20% for all the heads on a circuit. In this example, if we are designing for 30psi at head #1, then the pressure at head #3 must be at least 24psi. So the size of the pipe from the valve to the 1st head must be sized so that the pressure is still about 30psi (after taking all the other pressure losses into account, such as water meter, backflow preventer, mainline, and valve). Then the combined pressure loss from head 1 to head 2 plus head 2 to head 3 must be less than 6psi.
an example, lest assume you have 3 heads on a strip. T

<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by drpete3</i>
<br />I am no expert but I would think the best situation would to use the 1.25 pipe all the way then use funny pipe for the last foot prior to the head because you loose more pressure the longer the run and that phenomonon is amplified with smaller diameter pipe. Correct? Or am I missing somthing?
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How do you guys run pipe that runs over 100'? keep the same diameter or telescope to smaller diameter piping. I really would like to know because my design book shows nothing on this really. I'm also trying to avoid water hammer by going with larger diameter piping. let me know please.

I am no expert but I would think the best situation would to use the 1.25 pipe all the way then use funny pipe for the last foot prior to the head because you loose more pressure the longer the run and that phenomonon is amplified with smaller diameter pipe. Correct? Or am I missing somthing?

Why would you ever want to decrease pipe diameter because of a long run? Longer run means more resistance = less pressure. The larger the pipe, the less pressure loss.
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Exactly. So in order to keep that pressure up, you decrese the pipe size. It's actually the larger the pipe, the more flow, less pressure. The smaller the pipe, the more pressure, less flow.

Why would you ever want to decrease pipe diameter because of a long run? Longer run means more resistance = less pressure. The larger the pipe, the less pressure loss.
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Should i stay with 1 1/4" pipe for the long runs then or step up pipe sizes gradually?

Need more help expanding my irrigation system..

first off, let me say thanks to those that helped me get my irrigation system off the ground and working properly for my two front yard zones. What i would like to do now is expand into the three other areas of my yard (two sides and backyard). For the backyard, should I just put another manifold in my backyard and run a 1 1/4" mainline to it (from my first manifold in the front yard) and be able to place other valves for the backyard in this manifold box? The first manifold box has space for like one or two more valves, i figure I can run these for the left side of my yard (plants and grass).

My longest runs will be at least 200' i'm sure. Should I telescope to my 2nd manifold from 1 1/4" to 1" pipe if runs become longer than 100' or how would you guys do it? What about my laterals to the heads - 1"?

One other thing, all my wiring is pulled to the first manifold box for future expansion for other valves as well. Should I grease cap some of these connections together in order to run to the 2nd manifold box? Any quick help appreciated.